Controlling static charge is an important issue in semiconductor manufacturing because of its significant impact on the device yields. Device defects caused by electrostatically attracted foreign matter and electrostatic discharge events contribute greatly to overall manufacturing losses.
Many of the processes for producing integrated circuits use non-conductive materials which generate large static charges and complimentary voltage on wafers and devices.
Air ionization is the most effective method of eliminating static charges on non-conductive materials and isolated conductors. Air ionizers generate large quantities of positive and negative ions in the surrounding atmosphere which serve as mobile carriers of charge in the air. As ions flow through the air, they are attracted to oppositely charged particles and surfaces. Neutralization of electrostatically charged surfaces can be rapidly achieved through the process.
Air ionization may be performed using electrical ionizers which generate ions in a process known as corona discharge. Electrical ionizers generate air ions through this process by intensifying an electric field around a sharp point until it overcomes the dielectric strength of the surrounding air. Negative corona occurs when electrons are flowing from the electrode into the surrounding air. Positive corona occurs as a result of the flow of electrons from the air molecules into the electrode.
To achieve the maximum possible reduction in static charges from an ionizer of a given output, the ionizer must produce equal amounts of positive and negative ions. That is, the output of the ionizer must be "balanced." If the ionizer is out of balance, the isolated conductor and insulators can become charged such that the ionizer creates more problems than it solves. Ionizers may become imbalanced due to power supply drift, power supply failure of one polarity, contamination of electrodes, or degradation of electrodes. In addition, the output of an ionizer may be balanced, but the total ion output may drop below its desired level due to system component degradation.
Accordingly, ionization systems incorporate monitoring, automatic balancing via feedback systems, and alarms for detecting uncorrected imbalances and out-of-range outputs. Most feedback systems are entirely or primarily hardware-based. Many of these feedback systems cannot provide very fine balance control, since feedback control signals are fixed based upon hardware component values. Furthermore, the overall range of balance control of such hardware-based feedback systems may be limited based upon the hardware component values. Also, many of the hardware-based feedback systems cannot be easily modified since the individual components are dependent upon each other for proper operation.
A charged plate monitor is typically used to calibrate and periodically measure the actual balance of an electrical ionizer, since the actual balance in the work space may be different from the balance detected by the ionizer's sensor.
The charged plate monitor is also used to periodically measure static charge decay time. If the decay time is too slow or too fast, the ion output may be adjusted by increasing or decreasing the preset ion current value. This adjustment is typically performed by adjusting two trim potentiometers (one for positive ion generation and one for negative ion generation). Periodic decay time measurements are necessary because actual ion output in the work space may not necessarily correlate with the expected ion output for the ion output current value set in the ionizer. For example, the ion output current may be initially set at the factory to a value (e.g., 0.6 .mu.A) so as to produce the desired amount of ions per unit time. If the current of a particular ionizer deviates from this value, such as a decrease from this value due to particle buildup on the emitter of the ionizer, then the ionizer high voltage power supply is adjusted to restore the initial value of ion current.
A room ionization system typically includes a plurality of electrical ionizers connected to a single controller. FIG. 1 (prior art) shows a conventional room ionization system 10 which includes a plurality of ceiling-mounted emitter modules 12.sub.1 -12.sub.n (also, referred to as "pods") connected in a daisy-chain manner by signal lines 14 to a controller 16. Each emitter module 12 includes an electrical ionizer 18 and communications/control circuitry 20 for performing limited functions, including the following functions:
(1) TURN ON/OFF; PA1 (2) send an alarm signal to the controller 16 through a single alarm line within the signal lines 14 if a respective emitter module 12 is detected as not functioning properly. PA1 (1) Detect an out-of-range parameter via a charged plate monitor; PA1 (2) Climb up on a ladder and adjust balance and/or ion output current potentiometer settings; PA1 (3) Climb down from the ladder and remove the ladder from the measurement area. PA1 (4) Read the new values on the charged plate monitor; PA1 (5) Repeat steps (1)-(4), if necessary.
One significant problem with the conventional system of FIG. 1 is that there is no "intelligent" communication between the controller 16 and the emitter modules 12.sub.1 -12.sub.n. In one conventional scheme, the signal line 14 has four lines; power, ground, alarm and ON/OFF control. The alarm signal which is transmitted on the alarm line does not include any information regarding the identification of the malfunctioning emitter module 12. Thus, the controller 16 does not know which emitter module 12 has malfunctioned when an alarm signal is received. Also, the alarm signal does not identify the type of problem (e.g., bad negative or positive emitter, balance off). Thus, the process of identifying which emitter module 12 sent the alarm signal and what type of problem exists is time-consuming.
Yet another problem with conventional room ionization systems is that there is no ability to remotely adjust parameters of the individual emitter modules 12, such as the ion output current or balance from the controller 16. These parameters are typically adjusted by manually varying settings via analog trim potentiometers on the individual emitter modules 12. (The balances on some types of electrical ionizers are adjusted by pressing (+)/(-) or UP/DOWN buttons which control digital potentiometer settings.) A typical adjustment session for the conventional system 10 having ceiling mounted emitter modules 12 is as follows:
The manual adjustment process is time-consuming and intrusive. Also, the physical presence of the operator in the room interferes with the charge plate readings.
Referring again to FIG. 1, the signal lines 14 between respective emitter modules 12 consist of a plurality of wires with connectors crimped, soldered, or otherwise attached, at each end. The connectors are attached in the field (i.e., during installation) since the length of the signal line 14 may vary between emitter modules 12. That is, the length of the signal line 14 between emitter module 12.sub.1 and 12.sub.2 may be different from the length of the signal line 14 between emitter module 12.sub.3 and 12.sub.4. By attaching the connectors in the field, the signal lines 14 may be set to exactly the right length, thereby resulting in a cleaner installation.
One problem which occurs when attaching connectors in the field is that the connectors are sometimes put on backwards. The mistake may not be detected until the entire system is turned on. The installer must then determine which connector is on backwards and must fix the problem by rewiring the connector.
The conventional room ionization system 10 may be either a high voltage or low voltage system. In a high voltage system, a high voltage is generated at the controller 16 and is distributed via power cables to the plurality of emitter modules 12 for connection to the positive and negative emitters. In a low voltage system, a low voltage is generated at the controller 16 and is distributed to the plurality of emitter modules 12 where the voltage is stepped up to the desired high voltage for connection to the positive and negative emitters. In either system, the voltage may be AC or DC. If the voltage is DC, it may be either steady state DC or pulse DC. Each type of voltage has advantages and disadvantages.
One deficiency of the conventional system 10 is that all emitter modules 12 must operate in the same mode. Thus, in a low voltage DC system, all of the emitter modules 12 must use steady state ionizers or pulse ionizers.
Another deficiency in the conventional low voltage DC system 10 is that a linear regulator is typically used for the emitter-based low voltage power supply. Since the current passing through a linear regulator is the same as the current at its output, a large voltage drop across the linear regulator (e.g., 25 V drop caused by 30 V in/5 V out) causes the linear regulator to draw a significant amount of power, which, in turn, generates a significant amount of heat. Potential overheating of the linear regulator thus limits the input voltage, which in turn, limits the amount of emitter modules that can be connected to a single controller 16. Also, since the power lines are not lossless, any current in the line causes a voltage drop across the line. The net effect is that when linear regulators are used in the emitter modules 12, the distances between successive daisy-chained emitter modules 12, and the distance between the controller 16 and the emitter modules 12 must be limited to ensure that all emitter modules 12 receive sufficient voltage to drive the module-based high voltage power supplies.
Accordingly, there is an unmet need for a room ionization system which allows for improved flexibility and control of, and communication with, emitter modules. There is also an unmet need for a scheme which automatically detects and corrects the miswire problem in an easier manner. There is also an unmet need for a scheme which allows individualized control of the modes of the emitter modules. The present invention fulfills these needs.